luci-mod-admin-full: limit interface name length to 15 chars

Limit the name of a new interface to 15 characters.
Add a note about the maximum length and the automatic protocol/bridge
prefixes (br-, 6in4-, pppoe- etc.).

Reference to:
https://dev.openwrt.org/ticket/20380
https://github.com/openwrt/luci/issues/507

There is a 15 character limit to the "real" interface name,
enforced both in the firewall and dnsmasq. The real interface name
includes the possible prefix "br-", "6in4-" etc. Example of an error:
  interface name `br-lan_protected' must be shorter than IFNAMSIZ (15)

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>